Compares two input strings using the Double Metaphone algorithm. An optional threshold parameter can be modified to adjust the tolerance for matching.

The Double Metaphone algorithm processes an input string render a primary and secondary spelling for it. For English language words, the algorithm removes silent letters, normalizes combinations of characters to a single definition, and removes vowels, except from the beginnings of words. In this manner, the algorithm can normalize inconsistencies between spellings for better matching. For more information, see https://en.wikipedia.org/wiki/Metaphone.

Tip: This function is useful for performing fuzzy matching between string values, such as between potential join key values.

Source values can be string literals, column references, or expressions that evaluate to strings.

Output: Generates a new compare_weak column containing the comparison of string1 and string2 column values using the Double Metaphone algorithm. The 'weak' parameter input means that only the secondary encodings for each input must match.